1. Field of the Invention
The present invention relates to volume control circuits and, more particularly, to a touch sensitive volume control circuit that is adapted for use in the small confines of a hearing aid.
2. Description of the Prior Art
Touch sensitive control systems are widely available today. They are used in microwave ovens, elevator buttons and remote controls. There are several variations that are used for volume control. One variation uses electronics to control a relay that activates a motor, which in turn adjusts an electromechanical volume control. Many other volume control systems are purely electronic and use complex circuits employing pulse width modulation techniques or electronic counters to set and hold the loudness level at a desired setting. The microwave oven touch sensors are generally for digital control, having only off and on functions, and are not designed to provide continuously variable level control.
Prior art circuits generally have a complex design with a complicated configuration of component parts, requiring them to occupy a relatively large space and consume high amounts of power. Such systems are not capable of being fitted within small spaces as necessary for use in conjunction with a hearing aid, and are not able to operate for long periods of time from a battery.
One example of a prior art touch sensitive control circuit is taught in U.S. Pat. No. 3,691,298 where the circuit uses the touch of a finger to charge a capacitor connected to the gate of a field effect transistor (FET). The drain of the FET is connected to the positive terminal of a power supply. The source of the FET is connected to a resistor that is connected to ground. Varying the gate voltage varies the drain to source resistance of the FET. This change in resistance causes a change in the voltage across the resistor connected to ground. The resistor voltage is used to set the gain for an amplifier circuit.
However, the use of a voltage to control an amplifier's gain does not adequately provide accurate touch sensitive volume control for miniaturized applications or where power requirements are low such as use in a hearing aid. This is because the amplifiers used in the patented circuit require more space than a hearing aid will allow and have higher power requirements and supply voltage limits than available in hearing aids.